The embryonic vertebrate heart is composed of 2 major chambers, a ventricle and an atrium, each with a characteristic morphology that defines its functional capacity. Congenital heart defects are often associated with dysmorphic cardiac chambers, but the regulation of cardiac chamber morphogenesis is not well understood. Cardiac chamber morphogenesis can be divided into 2 major phases: heart tube assembly, in which bilateral precursor populations unite to form a tube, and chamber emergence, in which this simple cylinder transforms into a series of morphologically discrete chambers. The long-term goal of our research is to identify the cellular and molecular mechanisms that regulate these 2 phases of chamber morphogenesis. Using the zebrafish as a model organism, we can combine embryologic and genetic approaches, using high-resolution live imaging to determine how key genes influence cardiomyocyte behavior. Our preliminary time-lapse analyses of heart tube assembly suggest that this process is driven by multiple genes that collaborate to regulate regional differences in directed cardiomyocyte movements. Additionally, our initial morphometric analyses of chamber emergence suggest that this process is governed. by both intrinsic and extrinsic factors that control regional changes in cardiomyocyte morphology. Finally, our recent studies of novel mutations suggest that the slow fuse and change of heart genes are critical regulators of heart tube assembly and chamber emergence, respectively. Building on this foundation, we propose to test (1) how cardiomyocyte movements are regulated by a critical mass of cells, formation of polarized epithelia, interactions with endoderm and endocardium, and slow fuse function, and (2) how cardiomyocyte morphology is regulated by hemodynamics, sarcomere integrity, interactions with the endocardium, and change of heart function. Together, these studies will reveal essential regulatory mechanisms of cardiogenesis and also enrich our understanding of general paradigms for organ formation.